1. Field of the Invention
This invention relates to the field of radiation detectors and, more particularly, to electromagnetic radiation sensors having a self-test capability incorporated therein.
2. Description of the Prior Art
There are numerous instances in the prior art where it is desired to detect electromagnetic radiation in the optical range. Typically photodetectors, which may be selected for their response sensivity to optical radiation in a specified frequency band (such as the infrared), are mounted to receive incident radiation. The output of the photodetector, which may be an electrical signal corresponding to the incident radiation, is applied to suitable electrical circuitry for processing.
In certain applications of such systems, it is desirable to stimulate a detector optically in order to generate some kind of system response. For example, to generate a built-in test equipment (BITE) capability for an optical sensor, it would be advantageous to provide an optical stimulus, rather than an electrical stimulus, since the optical stimulus can provide a better functional test of the optical detector element.
It has become customary in the manufacture of low cost detectors and emitters to mount the detector or emitter "chip" to small headers, which are small standard transistor mounting devices. These headers are produced in large volume and therefore are very economical to use. They typically incorporate lead feed-throughs and high quality glass-metal seals. An optical detector (e.g., a silicon photodiode) can be very readily mounted on such a header, as can also an optical emitter (e.g., a GaAs infrared emitting diode--IRED). However, when both the detector and emitter are mounted on such a header in conventional fashion, as would be required for low cost construction, the emitter would be aimed in the same direction as the detector; that is, 180.degree. away from the direction of the optical radiation input to the detector. In practice, the detector could still receive a small amount of the radiation from the emitter, due to stray reflection from various parts of the detector/emitter package. However, this small amount of optical coupling would imply that large drive currents would be required to the emitter to couple a reasonable amount of radiation onto the detector. Also, it would be difficult to control the coefficient of coupling, since the arrangement depends upon uncontrolled stray reflection.
Devices are known which provide for optical coupling between the emitter and detector in a single device. Examples of such may be found in U.S. Pat. Nos. 3,914,137 of Huffman et al, 4,124,860 of Johnson, and 4,160,308 to Courtney et al. These devices are optically coupled isolators which are used for coupling signals between separate electrical circuits in which it is essential that there be no electrical interconnection. For example, these optically coupled isolators are particularly useful in completing the control link between microprocessor circuitry used to control a plant production line and the specific motor power circuits for a conveyor system. Such devices are not, however, applicable to the need to which the present invention is directed, since they do not incorporate an optical detector which is responsive to external radiation while at the same time being coupled to receive optical stimulus from an associated emitter for test purposes. Quite the contrary, in such optically coupled isolator devices it is essential that the optical detectors therein be immune to extraneous radiation. Thus, while the light-emitting and photo-sensitive chips of the device disclosed by Johnson are mounted on a substrate which is mounted on a conventional header with both chips being encapsulated in optically transparent material in a hemispherical shape for maximum internal reflection of light, an encapsulation cap, not shown in the drawing of the Johnson patent, shields the entire structure from ambient light. Similarly, in the device disclosed by Courtney et al, although the emitter and detector elements face each other, protection from ambient light is provided by encapsulating the entire structure in a moulding compound. The device of Huffman et al uses a metal shield to block ambient light. Such devices, therefore, are not useful as detectors of external radiation.